"The intention of the SAF in this move is to try to disrupt the referendum process," he said in a statement.

The leaders of north and south Sudan have accused each other of building up troops in the border region ahead of the January 9 poll in which southerners will be asked if they want independence from the north.

While the south is seen likely to vote for secession, the north would like to keep the country whole.

If confirmed, the attack would be the second time this month the north has bombed targets in the south.

On the first occasion, the southern army said northern forces accidentally dropped a bomb on its territory while fighting rebels from Darfur near the north-south border.

But the Sudanese army has denied it carried out the latest attack on the SPLA military base.

"This is absolutely not true. We have not attacked anywhere near the border," al-Sawarmi Khaled, the SAF spokesman, said.

Darfur allegations

Meanwhile, in another sign of the increasing tensions ahead the referendum, a senior northern figure said the south had declared war by supporting rebels who have been fighting government forces and their allies in the western Darfur region.

"If you are accommodating these forces in the south, you are supplying these forces with weapons, logistics, petrol and cars," Mandour al-Mahdi, a senior official from the northern National Congress Party, told the Reuters news agency.

"We think that this is a declaration of war against the north of the country."